Method and apparatus for treating amonia-containing effluent water

ABSTRACT

Disclosed are a method and an apparatus for treating an effluent containing ammonia in which method and apparatus N 2 O concentration in the gas at the outlet of a catalyst tower does not rise to a high level even when the NH 3  concentration in the effluent was reduced and the amount of hazardous substances formed is small; in the method and apparatus, an NH 3 -containing effluent A and a carrier gas (steam C and combustion gas F) are contacted in stripping tower 7 to transfer the NH 3  from the NH 3 -containing effluent to a gas phase, the gas containing the generated NH 3  is heated with pre-heater  19  and then contacted with catalyst layer  13  placed in catalyst tower  12  to decompose the NH 3  into nitrogen and water; and at that time, the oxygen concentration in the gas to be introduced into catalyst tower  12  and the N 2 O concentration in the gas discharged from catalyst tower  12  are determined by measuring instruments  21  and  22 , respectively, and the oxygen concentration in the gas to be introduced into catalyst tower  12  is adjusted by adjusting valve  17  so that the N 2 O concentration becomes within a prescribed range.

TECHNICAL FIELD

[0001] The present invention relates to a method for treating aneffluent (waste water) containing ammonia(NH₃). More specifically, thepresent invention relates to a method and an apparatus for treating anNH₃-containing effluent by which method or apparatus the ammoniacontained in the effluent discharged especially from a thermal powerplant is efficiently converted into nitrogen (N₂) and water (H₂O) tomake the ammonia harmless by a stripping method.

BACKGROUND ART

[0002] In recent years, there has been a growing concern to theconservation of global environment, and regulations againstover-fertilization of sea areas have been enforced. Thus, thedevelopment of a new technology for removing nitrogen from an effluenthas been sought. In answer to such request, the removal of the nitrogencontained in an effluent has been conducted from some time ago mainly bythe following methods:

[0003] 1) Biological denitrification method: Method in which an organicnitrogen contained in water is converted into an inorganic nitrogen torender the organic nitrogen harmless by using a bacterium.

[0004] 2) Discontinuous NH₃ decomposition method with chlorine: Methodin which NH₃ is oxidized to decompose by using sodium hypochlorite.

[0005] 3) Ion exchange method: Method in which NH₃ is adsorbed on azeolite through an ion exchange.

[0006] 4) Ammonia stripping method: Method in which NH₃ is diffused orevaporated from an NH₃-containing effluent into the atmosphere by usingair or steam.

[0007] When the BOD (biochemical oxygen demand) of an effluent is high,biological denitrification method 1) described above is used. On theother hand, in the case where an effluent in which most of nitrogen isin a form of ammonia nitrogen such as ammonia and ammonium ion is to betreated, for instance, when an effluent from a process in a chemicalfactory or an effluent once-subjected to a post-treatment is the objectof the treatment, method 2), 3) or 4) is used.

[0008] However, the conventional methods described above have theproblems as follows:

[0009] In the method 1), the size of a reaction bath necessary for thetreatment becomes large since the rate of a biological reaction is slow,and thus a large space becomes necessary for placing the reaction bath.Besides, the method 1) raises the problem that excess amount of a sludgeis produced. Method 2) causes the problem that a treatment of remainingchlorine becomes necessary and organic chlorine compounds are formed,since the addition of sodium hypochlorite in an amount more than thatstoichio-metrically required is necessary for completely removing theammonia. In the method 3), a secondary effluent containing ammonium ionat a high concentration is produced at the time of regenerating a usedzeolite and thus a treatment of the secondary effluent becomesnecessary. Further, the method 4) has the problem that an NH₃-containinggas is diffused or dissipated into the atmosphere after the NH₃ wastransferred into a gas phase and causes a secondary pollution.

[0010] Among the methods described above, method 4) is advantageouscompared with other methods since the treatment of an effluent iscomparatively simple and the costs of equipments and operations aresmall. Accordingly, a combination in which the method 4) is performed incombination with another method which can be used for oxidizing todecompose the NH₃ contained at a high concentration in a gas separatedfrom an effluent, by using a catalyst, to make the NH₃ contained in theeffluent harmless as the result of the combination has been adopted evenin current night-soil treatment facilities. However, in such a strippingand catalytically oxidizing process, it is necessary to install acatalyst tower for reducing NOx in addition to a catalyst tower foroxidizing NH₃ since a large quantity of NOx is generated at the time ofthe oxidation of NH₃. Further, according to the investigations by thepresent inventors, it has been found out anew that a large quantity ofN₂O is also produced in this process at the time of oxidizing the NH₃.Like CO₂, N₂O is a substance contributing to the global warming.Accordingly, it is dangerous to the global environment that a largequantity of N₂O is diffused into the atmosphere, in the same extent asNH₃ is diffused as it is. Thus, the diffusion of N₂O is alsoundesirable.

[0011] As described above, treatments of NH₃-containing effluents inconventional technology have many problems and some of the treatmentshad a problem that they might become sources from which varioussecondary pollution substances are produced anew.

DISCLOSURE OF THE INVENTION

[0012] Subject of the present invention is to propose a method and anapparatus for treating an NH₃-containing effluent in which method andapparatus the amount of secondary pollution substances formed is reducedand the amount of utilities such as steam to be used can also bereduced.

[0013] In order to achieve the subject described above, the method andapparatus of the present invention are summarized as follows:

[0014] (1) A method for treating an ammonia-containing effluentcomprising a stripping step in which the ammonia (NH₃) contained in theNH₃-containing effluent is transferred with a carrier gas from theeffluent into a gas phase, a step for adding an oxygen-containing gas tothe NH₃-containing gas produced at the stripping step, and an NH₃decomposing step in which the oxygen-containing gas and theNH₃-containing gas are contacted with one or more kind of catalysts usedfor decomposing NH₃, at a prescribed temperature to decompose the NH₃into nitrogen and water, the concentration of oxygen in the gas mixtureintroduced into the NH₃ decomposing step being adjusted.

[0015] (2) The method for treating an NH₃-containing effluent recited inparagraph (1) above wherein the method further comprises a step by whicha part of the gas resulted in the NH₃ decomposing step is dischargedoutside the effluent treating system and the remaining part of the gasresulted in the decomposing step is recycled as a part of the carriergas to be used in the stripping step.

[0016] (3) The method for treating an NH₃-containing effluent recited inparagraph (1) or (2) above wherein the concentration of oxygen in thegas mixture to be introduced into the NH₃ decomposing step is adjustedto a value within the range of 2 to 15%.

[0017] (4) The method for treating an NH₃-containing effluent recited inany one of paragraphs (1) to (3) above wherein the concentration ofoxygen in the gas mixture to be introduced into the NH₃ decomposing stepis adjusted so that the concentration of the N₂O in the gas resulted inthe NH₃ decomposing step becomes a value within a prescribed range.

[0018] (5) The method for treating an NH₃-containing effluent recited inany one of paragraphs (1) to (4) above wherein the catalyst used fordecomposing NH₃ comprises a first component having an activity ofreducing nitrogen oxides with NH₃ and a second component having anactivity of forming nitrogen oxides (NOx) from NH₃.

[0019] (6) The method for treating an NH₃-containing effluent recited inany one of paragraphs (1) to (5) above wherein the catalyst used fordecomposing NH₃ comprises, as a first component, an oxide of titanium(Ti) and an oxide of one or more elements selected from the groupconsisting of tungsten (W), vanadium (V), and molybdenum (Mo), and, as asecond component, a silica, zeolite, and/or alumina having one or morenoble metals selected from the group consisting of platinum (Pt),iridium (Ir), rhodium (Rh), and palladium (Pd) supported thereon.

[0020] (7) The method for treating an NH₃-containing effluent recited inany one of paragraphs (1) to (5) wherein the catalyst used fordecomposing NH₃ is a zeolite or comprises, as a main component, azeolite.

[0021] (8) The method for treating an NH₃-containing effluent recited inany one of paragraphs (1) to (4) above wherein the concentration ofoxygen in the gas mixture to be introduced into the NH₃ decomposing stepis adjusted so that the concentration of the NH₃ remaining in the gasresulted in the NH₃ decomposing step becomes a value within a prescribedrange, while using the concentration of the NH₃ remaining in the gasresulted in the NH₃ decomposing step as an index, instead of theconcentration of the N₂O in the gas.

[0022] (9) The method for treating an NH₃-containing effluent recited inparagraph (8) above wherein the concentration of the NH₃ remaining inthe gas resulted in the NH₃ decomposing step is higher than 50 ppm.

[0023] (10) The method for treating an NH₃-containing effluent recitedin paragraph (8) or (9) above wherein the gas pressure in the effluenttreating system is controlled to a prescribed value so that the amountof a part of the gas resulted in the NH₃ decomposing step and dischargedoutside the system becomes equal to the increase of the total amount ofthe gas including the amount of the oxygen-containing gas supplied intothe system.

[0024] (11) The method for treating an NH₃-containing effluent recitedin any one of paragraphs (8) to (10) above wherein the method furthercomprises a step for removing ammonia from a part of the gas resulted inthe NH₃ decomposing step after the part of the gas was dischargedoutside the system.

[0025] (12) The method for treating an NH₃-containing effluent recitedin any one of paragraphs (1) to (4) above wherein the step fordecomposing the NH₃ into nitrogen and water by contacting theNH₃-containing gas with a catalyst is

[0026] a step in which two or more kind of catalysts each having adifferent power for oxidizing NH₃ are used, and the NH₃-containing gasis contacted first with a catalyst having a relatively low power foroxidizing NH₃ to decompose a part of the NH₃ into nitrogen and water andthen with a catalyst having a relatively high power for oxidizing NH₃ todecompose the remaining part of the NH₃ into nitrogen and water, or

[0027] a step in which the NH₃-containing gas is contacted at the sametime with two or more kind of the catalysts to decompose the NH₃ intonitrogen and water.

[0028] (13) The method for treating an NH₃-containing effluent recitedin paragraph (12) above wherein two or more kind of the catalysts eachhaving a different power for oxidizing NH₃ comprise, as a firstcomponent, an oxide of titanium (Ti) and an oxide of one or moreelements selected from the group consisting of tungsten (W), vanadium(V), and molybdenum (Mo), and, as a second component, a silica, zeolite,and/or alumina having one or more noble metals selected from the groupconsisting of platinum (Pt), iridium (Ir), rhodium (Rh), and palladium(Pd) supported thereon, and the power for oxidizing NH₃ is adjusted bythe ratio of the content of the first component to that of the secondcomponent.

[0029] (14) The method for treating an NH₃-containing effluent recitedin paragraph (12) above wherein the catalyst having a relatively highpower for oxidizing NH₃ is a zeolite.

[0030] (15) An apparatus for treating an NH₃-containing effluentcomprising a stripping means for transferring the ammonia (NH₃)contained in the NH₃-containing effluent with a carrier gas from theeffluent into a gas phase, a means for adding an oxygen-containing gasto the NH₃-containing gas produced in the stripping means, an NH₃decomposing means by which the oxygen-containing gas and theNH₃-containing gas are contacted with one or more kind of catalysts usedfor decomposing NH₃, at a prescribed temperature to decompose the NH₃into nitrogen and water, and a means for adjusting the concentration ofoxygen in the gas mixture to be introduced into the NH₃ decomposingmeans.

[0031] (16) The apparatus for treating an NH₃-containing effluentrecited in paragraph (15) above wherein the apparatus further comprisesa means for determining the concentration of N₂O or NH₃ in the gasdischarged from the NH₃ decomposing means and controlling theconcentration to a value within a prescribed range.

[0032] (17) The apparatus for treating an NH₃-containing effluentrecited in paragraph (15) or (16) above wherein the catalyst used fordecomposing NH₃ comprises, as a first component, an oxide of titanium(Ti) and an oxide of one or more elements selected from the groupconsisting of tungsten (W), vanadium (V), and molybdenum (Mo), and, as asecond component, a silica, zeolite, and/or alumina having one or morenoble metals selected from the group consisting of platinum (Pt),iridium (Ir), rhodium (Rh), and palladium (Pd) supported thereon.

[0033] (18) The apparatus for treating an NH₃-containing effluentrecited in paragraph (15) or (16) above wherein the catalyst used fordecomposing NH₃ is a zeolite or comprises, as a main component, azeolite.

[0034] (19) The apparatus for treating an NH₃-containing effluentrecited in paragraph (15) above wherein the catalyst used fordecomposing NH₃ comprises

[0035] a catalyst in which one or more catalyst layers having arelatively low power for oxidizing NH₃ and one or more catalyst layershaving a relatively high power for oxidizing NH₃ are arranged in series,or

[0036] a plate-like catalyst in which catalyst layers having arelatively low power for oxidizing NH₃ and catalyst layers having arelatively high power for oxidizing NH₃ are arranged alternately in thedirection perpendicular to the direction of the gas flow with thesurfaces of the layers being held in parallel to the gas flow direction.

[0037] As specific examples of catalysts comprising a first componenthaving an activity of reducing nitrogen oxides with NH₃ and a secondcomponent having an activity of forming nitrogen oxides (NOx) from NH₃,catalysts comprising, as a first component, an oxide of titanium (Ti)and an oxide of one or more elements selected from the group consistingof tungsten (W), vanadium (V), and molybdenum (Mo), and, as a secondcomponent, a silica, zeolite, and/or alumina having one or more noblemetals selected from the group consisting of platinum (Pt), iridium(Ir), rhodium (Rh), and palladium (Pd) supported thereon can bementioned. Besides, catalysts substantially consisting of zeolite orcomprising, as a main component, zeolite have an effect of considerablyreducing the formation of NOx or N₂O.

[0038] By changing the ratio of the first component to the secondcomponent in the catalyst described above, it is possible to adjust theconcentrations of NH₃ and NOx in the gas resulted in the NH₃decomposition. For instance, when the ratio of the second component wasreduced, the ratio of decomposition of NH₃ is slightly lowered, but theconcentration of NOx in the resulting gas is considerably reduced.

[0039] In order to transfer the NH₃ contained in an NH₃-containingeffluent from the effluent into a gas phase, a method in which the NH₃contained in the effluent is stripped into the gas phase, specifically,for example, (a) a method in which a carrier gas is blown into theeffluent, and (b) a method in which the effluent is sprayed in a carriergas are used. When the effluent has a pH of 10 or higher, the strippingis performed as it is. On the other hand, when the effluent has a pH oflower than 10, an alkali such as sodium hydroxide and calcium hydroxide(slaked lime) is first added to the effluent to make its pH 10 orhigher, and then the effluent is contacted with air to diffuse orevaporate the NH₃ into the air by using the air as a carrier gas. As thecarrier gas, steam can be used in place of air. The term “carrier gas”as used herein generically means a gas which gas can diffuse orevaporate ammonia from the effluent.

[0040] An NH₃-containing gas is preheated at the time when the gas isintroduced into a stripping tower or catalyst tower, when necessary.Preheating may be conducted by a usual method, for example, by heatingwith a burner or heat exchange with a gas at a high temperature such assteam or a gas discharged from a catalyst device. When the gas iscirculated in the method and apparatus of the present invention, it ispreferable to use a method in which the composition of the gas,especially the concentration of oxygen in the gas is not changed. (As anexample, a method using an indirect heat exchange is preferable.)

[0041] In the case where an NH₃ decomposing catalyst having adenitrating function is used, it is important to control the temperatureof a catalyst layer in a catalyst tower in the range of 250 to 450° C.,preferably in the range of 350 to 400° C. In the case where a zeolitetype catalyst is used, it is preferable to maintain the temperature of acatalyst layer in the range of 450 to 600° C. In any case, it issatisfactory that a suitable temperature is selected based on theperformances of a catalyst.

[0042] The term “NH₃-containing effluent” used herein means an effluentcontaining ammonia nitrogen, such as an effluent discharged from a draintreating plant or sewerage treating facility, and an effluent dischargedfrom a dry type electrostatic precipitator or wet type desulfurizingapparatus installed for removing respectively combustion ashes or SO₂gas each contained in an exhaust gas discharged from a thermal powerplant having a coal firing boiler or oil firing boiler. Also, the term“NH₃-containing effluent” includes effluents which contain an nitrogenconverted into ammonia nitrogen by a pretreatment, such as an effluentin which an organic nitrogen originally contained in the effluent wasdecomposed into strippable ammonia nitrogen by a general biologicaltreatment, and an effluent containing NH₃ at a high concentration anddischarged at the time of the regeneration of a zeolite in aconventional ion exchange method used in various fields of industry.

BRIEF DESCRIPTION OF THE DRAWINGS

[0043]FIG. 1 is a flow diagram for illustrating an embodiment of themethods for treating an NH₃-containing effluent and the arrangements ofdevices in the apparatuses of the present invention.

[0044]FIG. 2 is a line graph showing the relation between theconcentration of oxygen gas introduced into a catalyst tower, andammonia decomposition ratio and N₂O concentration in the resulting gaswhen an ammonia-containing effluent was treated in the embodiment shownin FIG. 1.

[0045]FIG. 3 is a flow diagram similar to that of FIG. 1, showinganother embodiment of the present invention.

[0046]FIG. 4 is a flow diagram similar to that of FIG. 1, showing stillanother embodiment of the present invention.

[0047]FIG. 5 is a line diagram showing the relation between theconcentration of ammonia and the concentration of the sum of NOx and N₂Oin the gas discharged from a catalyst tower in the embodiment of thepresent invention shown in FIG. 4.

[0048]FIG. 6 is a flow diagram showing the method and the apparatus ofthe present invention in the case where two catalyst towers are used.

[0049]FIG. 7 is a diagram for illustrating the structure of an NH₃decomposing catalyst having a denitrating function and used in thepresent invention, and briefly illustrating reactions performed therein.

[0050]FIG. 8 is a diagram for illustrating the effects by which ammoniais removed when a catalyst having a relatively low power for oxidizingNH₃ and/or a catalyst having a relatively high-power for oxidizing NH₃is used.

[0051]FIG. 9 is a diagram for illustrating the structure of a catalysttower when two of the catalysts shown in FIG. 8 are used in the tower.

[0052]FIG. 10 is a line graph showing the relation between the reactiontemperature and the concentration of the sum of NOx and N₂O contained inthe gas discharged from a catalyst tower when two of the catalysts shownin FIG. 8 were used.

[0053]FIG. 11 is a diagram of a catalyst device prepared by using twokinds of plate-like catalysts each comprising one type of catalyst, inthe case where two catalysts shown in FIG. 8 were used.

BEST MODE FOR CARRYING OUT THE INVENTION

[0054] Now, the embodiments of the present invention are described inmore detail with reference to drawings.

[0055] The model of a fine pore in an NH₃ decomposing catalyst having adenitrating function and used in the present invention is shown in FIG.7.

[0056] As demonstrated in FIG. 7, the fine pore has a structure in whichmicro-pores inherently contained in a porous silica exist at placeswithin a macro-pore formed by a component (first component) on thesurface of which NO is reduced by NH₃, and ultramicro-particles ofanother component (second component) having an activity of forming NOxfrom NH₃ are supported on the surface of the micro-pores of the silica.NH₃ diffuses within the macro-pore in a catalyst, the diffused NH₃ isoxidized on the second component to form NO according to the equation(1) described below, the NO collide with NH₃ adsorbed on the surface ofthe first component forming the macro-pore, in the course of diffusingoutside the catalyst, and the NH₃ is reduced down to N₂ according to theequation (2) described below. As a whole, the NH₃ is changed as shown byequation (3) described below.

NH₃+5/4O₂→NO+3/2 H₂O  (1)

NH₃+NO+1/4O₂→N₂+3/2H₂O  (2)

NH₃+3/4O_(2 →)1/2N₂+3/2H₂O  (3)

[0057] As described above, it is possible to reduce NH₃ to N₂ whilescarcely forming, as a final product, NO or N₂O which is generallyconsidered to be formed during the process of forming NO, when an NH₃decomposing catalyst having a denitrating function is used, since theoxidizing reaction of NH₃ and the reducing reaction of formed NO withNH₃ proceed within the catalyst.

[0058] Besides, even when a zeolite is used, the amount of NO or N₂Oformed is extremely small.

[0059] However, even in the case where such catalyst is used, aphenomenon in which the concentration of N₂O at the outlet of a catalysttower becomes slightly high when the concentration of NH₃ in an effluentwas high was observed. As a result of diligent investigations by thepresent inventors, it has been found out that the means described belowis effective to such increase of the concentration of N₂O at the outletof a catalyst tower.

[0060] When the concentration of oxygen in a gas within a catalyst tower(herein, the concentration of oxygen in a gas to be introduced into acatalyst tower) is low, ammonia decomposition ratio and theconcentration of the N₂O contained in the resulting gas decrease, butthe decrease in the concentration of N₂O occurs at a higherconcentration of oxygen than the oxygen concentration where the ammoniadecomposition ratio comes to decrease. In an example, whereas the oxygenconcentration at which the ammonia decomposition ratio begun to decreasewas 3% or lower, N₂O concentration begun to decrease when the oxygenconcentration became 10% or lower. Thus, it has been found out that theN₂O concentration can be reduced without lowering ammonia decompositionratio by maintaining the oxygen concentration at a proper value (herein,2 to 15%, preferably 2 to 10%).

[0061]FIG. 1 is a flow diagram showing a system of devices in the casewherein a method of the present invention for treating an NH₃-containingeffluent is applied to an effluent containing ammonia nitrogen at a highconcentration, for example, to an effluent discharged from a thermalpower plant.

[0062] As shown in FIG. 1, effluent A and alkali B are supplied to tank3 through pipe 1 and pipe 2, respectively, mixed within tank 3, and thenfed to pre-heater 5 with pump 4. The effluent A preheated up to about100° C. with pre-heater 5 is supplied to a top portion of strippingtower 7 through pipe 6. Within stripping tower 7, filler material 8 isplaced. Steam C and combustion gas F are supplied as carrier gasesthrough pipe 9 and pipe 16 connected to bottom portions of the tower,respectively, and rise within the tower while efficiently contactingwith effluent A within the tower to obtain a gas containing ammonia at ahigh concentration. The concentration of NH₃ in the gas thus obtained isusually a few thousands to a few tens of thousands ppm. To the gas thusobtained is a proper amount of air D supplied through pipe 18 byadjusting the opening of regulating valve 17 according to the signalsfrom a device for measuring oxygen concentration and a device formeasuring N₂O concentration both described below.

[0063] Another combustion gas F supplied from a combustion device (notshown in the drawing) is fed into pre-heater 19 through pipe 20, mixedtherein with the gas containing ammonia, heated up to a prescribedtemperature, and then introduced into catalyst tower 12. Near the inletof catalyst tower 12, device 21 for measuring oxygen concentration isinstalled, and the oxygen concentration in the gas is determined withthe device. The stripped ammonia contained in the gas is oxidized todecompose into N₂ and H₂O on catalyst (layer) 13, and then dischargedinto the atmosphere through pipe 14.

[0064] The concentration of N₂O at the outlet of the catalyst tower 12is determined with device 22 installed near the outlet of catalyst tower12 and used for measuring N₂O concentration, the determined value thusobtained and the determined value obtained by device 21 for measuringoxygen concentration are inputted into control unit 30, and the controlunit 30 controls the flow rate of air D to be supplied through pipe 18with regulating valve 17 according to the determined values. From pipe15 connected to a bottom portion of stripping tower 7, effluent E fromwhich ammonia was removed is discharged.

[0065] Two combustion gases F supplied through pipe 16 and pipe 20,respectively, may be gases other than combustion gases so far as thegases have high temperatures and low oxygen concentrations. The catalystused in catalyst tower 12 comprises a first component having an activityof reducing nitrogen oxides with NH₃ and a second component having anactivity of forming nitrogen oxides (NOx) from NH₃. Further, thereaction temperature in catalyst layer 13 at the time of operation is250 to 450° C., preferably 350 to 400° C.

[0066] According to the embodiment shown in FIG. 1, it is possible toremove ammonia at a high efficiency while suppressing the formation ofN₂O by controlling the amount of oxygen in the catalyst device. Besides,according to the embodiment shown in FIG. 3, heat loss released outsidethe system is reduced since the amount of the gas discharged outside thesystem is decreased in addition to the effect described above. Thus, asmall amount of heating energy is satisfactory in the pre-heater.

[0067]FIG. 3 is a diagram showing a system of devices in anotherembodiment of the present invention. Its difference from the system usedin the apparatus shown in FIG. 1 is that a part of the gas dischargedfrom catalyst tower 12 is returned to stripping tower 7 with fan 25through pipe 24 to employ as a part of a carrier gas thereby the amountof heating energy can be reduced.

[0068] That is, Gas G containing N₂ and H₂O formed by the decompositionof the ammonia leaves catalyst tower 12 and passes through pipe 24, apart of the gas is discharged through pipe 23 into the atmosphere, andthe remaining part of the gas is returned to stripping tower 7 with fan25. It is the same as in the case of FIG. 1 that the concentration ofN₂O at the outlet of catalyst tower 12 is determined by N₂Oconcentration measuring device 22 installed at a midway of pipe 24, thedetermined value thus obtained and the determined value obtained byoxygen concentration measuring device 21 are inputted into control unit30, and the control unit 30 controls the flow rate of the air suppliedthrough pipe 18, with regulating valve 17 according to the determinedvalues.

[0069] Whereas steam C and combustion gas F (supplied through pipe 16)are used as a stripping gas in the case of the apparatus shown in FIG.3, pipe 16 (for supplying combustion gas F) can be omitted, for example,by controlling the amount of the circulating gas from pipe 24, or byproviding a pre-heater at pipe 9.

[0070] Whereas N₂O concentration measuring device 22 is used in thesystem of devices shown in FIG. 1 and FIG. 3, it is not necessary toalways use the device 22. For instance, it is sufficient that the rangeof proper oxygen concentrations at which ammonia decomposition ratio ishigh and N₂O concentration in the gas discharged from a catalyst towercan be decreased lower than a prescribed value is ascertained in advanceand then the oxygen concentration is adjusted so as to become aconcentration within the ascertained range.

[0071]FIG. 4 shows the same diagram of the system of devices as shown inFIG. 3 with the exception that N₂O concentration measuring device 22 isomitted and pipe 16 for supplying combustion gas F as a stripping gas isomitted by providing pre-heater 40.

[0072] In the apparatuses described above, air D is added through pipe18 to the gas discharged from stripping tower 7 and containing ammoniaat a high concentration, and the gas mixture is introduced into catalysttower 12, after preheated up to a prescribed temperature with pre-heater19, when necessary. The amount of air D added through pipe 18 isadjusted so that the amount of oxygen contained in the air becomes equalto the amount consumed by the decomposition reaction performed oncatalyst 13. In this connection, it is possible to add oxygen gasinstead of air. The gas containing the stripped ammonia is contactedwith catalyst 13 used for decomposing NH₃ and having a denitratingfunction to oxidatively decompose the ammonia into N₂ and H₂O on thecatalyst 13 described above. The reaction temperature at this time incatalyst layer 13 is 250 to 450° C., preferably 350 to 400° C. in thecase of an NH₃ decomposing catalyst having a denitrating function, andpreferably 450 to 600° C. in the case of a zeolite type catalyst. Gas Gcontaining N₂ and H₂O formed by the decomposition of ammonia is returnedto stripping tower 7 with fan 25 as a part of a carrier gas through pipe24, after the gas temperature is raised with pre-heater 40, whennecessary. A part of gas G is discharged outside the system through pipe23 connected to pipe 24 at a position between fan 25 and pre-heater 40.It is sufficient that the amount of the gas discharged outside thesystem through pipe 23 is the same as the-amount of oxygen consumed incatalyst tower 12, specifically the same as the increased amount of agas such as air D added through pipe 18. In order to control the amountof the gas to be discharged through pipe 23, it is sufficient that thegas pressure within the system at a prescribed place is determined andthe gas is discharged so that the gas pressure at that place becomesconstant.

[0073] The moisture in gas G is condensed into water by cooling the gaswith a condenser (not shown in the drawings). A slight amount of NH₃contained in the gas may be recovered together with the condensed water.Alternatively, the NH₃ contained in the gas may be absorbed in a liquid(not shown in the drawings) containing an acid such as sulfuric acid bycontacting the discharged gas with the liquid. From pipe 15. connectedto a bottom portion of stripping tower 7, effluent E from which ammoniawas removed is discharged. While steam C supplied through pipe 9 isnecessary at the initial stage of operation, it becomes unnecessary whenthe temperature within stripping tower 7 became sufficiently high. Fan25 may be located at a place other than that shown in FIG. 4. Forinstance, the place may be the middle point between stripping tower 7and catalyst tower 12, but the location shown in FIG. 4 is preferablefrom the viewpoint of holding the inside of the catalyst tower at anegative pressure and preventing a possible gas leakage.

[0074] With respect to the composition of the gas in catalyst tower 12shown in FIG. 4 before and after the reaction, only the amounts of NH₃and O₂ contained in the gas before the reaction decrease during thereaction and an equal amount of N₂ and H₂O are formed. In general, thereis no case where the gas composition is largely changed by the reactionsince the NH₃ concentration in the gas to be treated is a few thousandsppm. Then, it becomes possible to circulate the gas once-subjected to anNH₃ decomposition reaction to use as a part of a carrier gas, byintroducing air in an amount commensurate with the amount of consumedoxygen in the system and taking the increased portion of the gas outsidethe system. By conducting such procedures, a heat source and relatedparts (in this case, one of combustion gases F and pipe 16 in FIGS. 1and 3) for preheating a carrier gas, and in some cases, a heat exchangerused when the gas discharged from a catalyst tower is preheated can bemade unnecessary.

[0075] Further, it becomes easy to remove a slight amount of NH₃contained in a discharged gas since the amount of the gas dischargedoutside the system becomes a few tenths to one hundredth of the amountin a conventional case. For instance, NH₃ can be recovered together withwater by lowering the temperature of the gas to condense the moisturecontained in the gas into water since NH₃ easily dissolves in water.Alternatively, it is possible to contact the discharged gas with aliquid containing an acid such as sulfuric acid to have the NH₃ in thegas absorbed in the liquid.

[0076] Still further, it is possible to decrease the concentrations ofNH₃ and NOx in the treated gas down to an extremely low level, since NOxconcentration reduces by using a device for measuring ammoniaconcentration in place of a device for measuring N₂O concentration usedin the system of devices shown in FIGS. 1 and 3, and conducting theoperation under the conditions wherein the NH₃ concentration in the gasat the outlet of a catalyst tower layer is increased up to a prescribedvalue.

[0077] An example of the relation between the NH₃ concentration and theconcentration of the sum of NOx and N₂O at the outlet of a catalystlayer is shown in FIG. 5. As a parameter changing the NH₃ concentrationof the abscissa, for example, the contact time between the gasdischarged from a stripping tower and a catalyst can be mentioned inaddition to the second component in the catalyst described above. Whenthe contact time between the gas discharged from a stripping tower and acatalyst was shortened by increasing the flow rate of the gas or byreducing the amount of the catalyst, the concentration of NH₃ at theoutlet of a catalyst layer increases. It is possible to make theconcentration of the sum of NOx and N₂O in the gas at the outlet of acatalyst tower lower than 1 ppm as shown by curve (a) in FIG. 5 byselecting an appropriate catalyst and maintaining the NH₃ concentrationin the gas at the outlet exit of a catalyst layer at 50 ppm or higher,preferably about 100 ppm. As such a catalyst, an ammonia decomposingcatalyst having a denitrating function can be mentioned. However, whenan appropriate catalyst is not selected, the concentration of the sum ofNOx and N₂O in the gas at the outlet of a catalyst layer can not belowered even if the NH₃ concentration in the gas at the outlet of thecatalyst layer was increased up to the highest as shown by curve (b) inFIG. 5.

[0078] Next, specific examples of the present invention are described.

EXAMPLE 1

[0079] Ammonium paratungstate ((NH₄)₁₀H₁₀.W₁₂O₄₆.6H₂O) in an amount of2.5 kg and 2.33 kg of ammonium metavanadate were added to 67 kg of aslurry of metatitanic acid (TiO₂ content: 30 wt %, SO₄ content: 8 wt %)and mixed by using a kneader. The paste thus obtained was granulated,dried, and then calcined at 550° C. for 2 hours. The granules thusobtained were ground to obtain powders as a first component of acatalyst. The powders had a composition of Ti/W/V=91/5/4 (ratio ofatoms). On the other hand, 500 g of fine powders of silica (produced byTomita Pharmaceuticals Co., Ltd.; trade name: Micon F) was added to 1 Lof 1.33×10⁻² wt % of chloroplatinic acid (H₂[PtCl₆].6H₂O), evaporated todryness on a sand bath, and then calcined at 500° C. for 2 hours in theair to prepare 0.01 wt % Pt.SiO₂ powders as a second component of thecatalyst.

[0080] Next, 5.3 kg of silica. alumina type inorganic fibers and 17 kgof water were added to the mixture of 20 kg of the first component and40.1 g of the second component, and kneaded to obtain a catalyst paste.Separately, a net-like product made of E glass fibers was impregnatedwith a slurry containing a titania, silica sol and polyvinyl alcohol,dried at 150° C. to prepare catalyst substrates. Between the catalystsubstrates was the catalyst paste described above held and they werepassed through press rollers to roll, thereby obtaining a plate-likeproduct. After the plate-like product was air-dried in the atmospherefor 12 hours, it was calcined at 500° C. for 2 hours to obtain an NH₃decomposing catalyst having a denitrating function. In the catalyst thusobtained, the ratio of the second component to the first component (thesecond component/the first component) was 0.2/99.8.

[0081] A test for treating an effluent was conducted by using thecatalyst obtained and the apparatus as shown in FIG. 1 under theconditions shown in Table 1. The effects of oxygen concentration in thegas in catalyst tower 12 on the decomposition ratio of ammonia and theconcentration of formed N₂O in the resulting gas are shown in FIG. 2. Aswill be seen from FIG. 2, the N₂O concentration was capable to beinglowered while maintaining a high ammonia decomposition ratio bymaintaining the oxygen concentration at the inlet of catalyst layer 13within the range of 5 to 10%.

[0082] Although it varies according to the catalyst to be used and thecomposition of the effluent to be treated, oxygen concentrationsappropriate for lowering the N₂O concentration while maintaining a highammonia decomposition ratio was 2 to 15% (more preferably 5 to 10%).TABLE 1 Item Condition Rate of treating effluent 1.6 L/h Amount of NH₄ ⁺in effluent 2,000 mg/L Gas flow rate at inlet of 0.8 m³/h catalyst layerGas composition NH₃: 10,000 ppm H₂O: 28% Air: the remainder Temperature400° C. Areal velocity 5 m/h

EXAMPLE 2

[0083] A test for treating an effluent was conducted by using thecatalyst prepared by the same way as in Example 1 and the apparatus asshown in FIG. 4 under the conditions shown in Table 2. In this test, theamount of the air supplied through pipe 18 was adjusted so that the flowrate of the gas discharged outside the system through pipe 23 became0.03 m³/h. While the concentration of NH₃ in gas G discharged was 100ppm, that of NO was 0.6 ppm, and that of N₂O was 18 ppm, the NH₃concentration was reduced down to lower than 0.1 ppm by contacting thedischarged gas with a diluted sulfuric acid. Besides, the amount ofsteam necessary for preheating a gas and liquid was kg/kg. TABLE 2 ItemCondition Rate of treating effluent 1.6 L/h Amount of NH₄ ⁺ in effluent2,000 mg/L Gas flow rate at inlet of 1.3 m³/h catalyst layer Gascomposition NH₃: 3,000 ppm H₂O: 28% Air: the remainder Temperature 350°C. Areal velocity 17 m/h

COMPARATIVE EXAMPLE 1

[0084] The first component and the second component of a catalyst wereprepared by the same way as in Example 1, and then 5.3 kg of silica .alumina type inorganic fibers and 17 kg of water were added to themixture of 20 kg of the first component and 202 g of the secondcomponent to obtain a catalyst paste. Separately, a net-like productmade of E glass fibers was impregnated with a slurry containing atitania, silica sol, and polyvinyl alcohol, and dried at 150° C. toprepare catalyst substrates. Between the catalyst substrates was thecatalyst paste described above held and they were passed through pressrollers to roll, thereby obtaining a plate-like product. After theplate-like product was air-dried in the atmosphere for 12 hours, it wascalcined at 500° C. for 2 hours to obtain an NH₃ decomposing catalysthaving a denitrating function. In the catalyst thus obtained, the ratioof the second component to the first component (the second component/thefirst component) was 1.0/99.0.

[0085] A test for treating an effluent was conducted by using thecatalyst thus obtained without circulating the treated gas under thesame conditions as in Example 1. As the result, the flow rate of the gasdischarged outside the system through pipe 14 was 1.3 m³/h, andconcentration of the NH₃ in the treated gas was 5 ppm, that of NO was 1ppm, and that of N₂O was 21 ppm. Besides, the amount of steam necessaryfor preheating a gas and liquid was 0.25 kg/kg.

[0086] From the comparison between Example 2 and Comparative Example 1,it is understood that in Example 2, the concentrations of NO and N₂O inthe gas at the outlet of the catalyst layer were low, the gas flow ratebecame lower than {fraction (91/40)}, and the amount of dischargedhazardous or poisonous gases was considerably reduced compared withComparative Example 1. Also, the amount of steam necessary for thepreheating becomes less than ½ of that in Example 2.

[0087] According to the embodiments shown in FIG. 4 and FIG. 6, the NH₃contained in an NH₃-containing effluent can be removed at a highefficiency at small amounts of NOx, NO, and N₂O to be formed. Further,the energy necessary for heating a liquid and a gas in the treatment ofan NH₃-containing effluent, and the amount of a gas containing ahazardous or harmful substance and discharged can considerably bereduced.

[0088] Even in the case where an NH₃ decomposing catalyst of the presentinvention having a denitrating function was used, a phenomenon in whichthe concentration of NO or N₂O in the gas at the outlet of a catalysttower became slightly high was observed when the NH₃ concentration inthe gas treated in a catalyst tower (catalyst tower 12 in FIGS. 1 to 4)was decreased to lower than a prescribed value. This is considered dueto the fact that when the reaction shown by the equation (1) describedabove was accelerated by increasing the amount of a second component ina catalyst in order to lower the NH₃ concentration in the treated gas,the concentration of NO becomes too high and the amount of NH₃ necessaryfor the reaction shown by the equation (2) described above becomesinsufficient. On the other hand, when the amount of a second componentin a catalyst was decreased, the concentrations of NOx and N₂O lower,but a problem that the concentration of NH₃ in the treated gas becomeshigh arises.

[0089] This fact is diagrammatically shown in FIG. 8. As shown in thedrawing, in the case of catalyst A (case 1 in FIG. 8), the NH₃concentration at the end of the treatment is high since the amount of asecond component in a catalyst is small, and in the case of catalyst B(case 2 in FIG. 8), the concentrations of NOx and N₂O at the end of thetreatment are high since the amount of a second component is large. Ithas now been found out that the following methods (case 3 and case 4 inFIG. 8) are effective for lowering simultaneously not only theconcentration of NH₃ but also the concentrations of NOx and N₂O in thegas at the time when the treatment with a catalyst was completed.

[0090] That is, an NH₃-containing gas heated up to a prescribedtemperature is contacted with a catalyst A having a relatively low powerof oxidizing NH₃ to decompose a part of the NH₃ described above intonitrogen and water. At this time, about 10% of the NH₃ contained in theoriginal gas remains unreacted, but the concentrations of NOx and N₂O inthe resulting gas are small. The NH₃ remained unchanged is thencontacted with catalyst B having a relatively high power of oxidizingNH₃ to decompose almost all remaining NH₃ into nitrogen and water (case3 in FIG. 8). When the gas was contacted with catalyst B having arelatively high power of oxidizing NH₃, NOx and N₂O tend to be formed.However, the concentrations of NOx and N₂O in the finally resulting gascan be decreased since the NH₃ concentration at the stage just prior tothe reaction by catalyst B is lowered down to about one tenth of theoriginal concentration by catalyst A. The term “power of oxidizing NH₃”used herein means the oxidizing power per unit volume of a catalyst.

[0091] Also, a method in which an NH₃-containing gas is contacted firstwith catalyst A having a relatively low power of oxidizing NH₃ todecompose a part of the NH₃ described above into nitrogen and water andthen contacted with zeolite catalyst C (case 4 in FIG. 8) is effective.Especially, this method is effective for reducing the concentration ofN₂O in the finally resulting gas since a certain type of zeolitecatalyst C has a function of decomposing N₂O.

[0092]FIG. 9 shows another example of catalyst tower 12 used in thesystem of devices shown in FIG. 1. In other words, FIG. 9 is a diagramfor illustrating a catalyst device in which catalyst layer 13A having arelatively low power of oxidizing NH₃ and catalyst layer 13B having arelatively high power of oxidizing NH₃ are packed in the direction ofgas flow within the tower. In the device of FIG. 9, an NH₃-containinggas adjusted to a prescribed temperature and oxygen concentration as inthe case of Example 1 in advance is introduced into catalyst tower 12,and contacted with catalyst layer 13A having a relatively low power ofoxidizing NH₃ to decompose a part of the NH₃ into nitrogen and water. Atthis time, about 10% of the NH₃ in the introduced gas remains unreacted.The NH₃ remained unreacted is then contacted with catalyst 13B having arelatively high power of oxidizing NH₃ to decompose almost all remainingNH₃ into nitrogen and water, and the gas thus resulted is dischargedinto the atmosphere through pipe 14. Further, device 60 for mixing a gasis provided between catalyst layer 13A and catalyst layer B, whennecessary, so that the gas once-subjected to the decomposition withcatalyst 13A is supplied to catalyst layer 13B after the gas was madeuniform. The reaction temperature in catalyst layers 13 at this time is250 to 500° C., preferably 350 to 450° C.

EXAMPLE 3

[0093] A test for treating an effluent was conducted in the same manneras in Example 2 while using the apparatus comprising a device systemsimilar to that shown in FIG. 4 with the exception that two catalysttowers of catalyst tower 50 for oxidizing NH₃ and catalyst tower 42 forreducing NOx were installed. FIG. 6 shows the device system used in thisexample.

[0094] In the apparatus shown in FIG. 6, air D was added to the exhaustgas discharged from stripping tower 7 and containing ammonia at a highconcentration. The mixed gas is preheated with pre-heater 19 up to aprescribed temperature, when necessary, and then introduced intooxidizing catalyst tower 50 in which an oxidizing catalyst as a secondcomponent is packed. However, a part of the (preheated) mixed gas issupplied through by-pass line 41 to reducing catalyst tower 42 in whicha reducing catalyst as a first component is packed, and NOx isreductively decomposed into NO and N₂. The amount of the gas to besupplied to reducing catalyst tower 42 through by-pass line 41 isadjusted by controlling the opening of valve 44 based on the indicatedvalue of ammonia meter 43 placed near the outlet of reducing catalysttower 42 so that the concentration of the NH₃ in the treated gas becomesa prescribed value (for example, a value within the range of 50 to 100ppm). On oxidizing catalyst 45 in oxidizing catalyst tower 50, NH₃ isdecomposed according the equation (3) described below. However,oxidizing reactions of the following equation (1) and equation (4) bothdescribed below occur at the same time to form NO and N₂O. Then, inorder to remove the NO formed, these gases are introduced together withNH₃-containing gas supplied through by-pass line 41 into reducingcatalyst tower 42, and NO is reduced to disappear on reducing catalyst46 according the following equation (2).

[0095] NH₃+5/4O₂→NO+3/2H₂O  (1)

NH₃+NO+1/4O₂→N₂+3/2H₂O  (2)

NH₃+3/4O₂→1/2N₂+3/2H₂O  (3)

NH₃+2O₂→1/2N₂O+3/2H₂O  (4)

[0096] The gas discharged from the outlet of reducing catalyst tower 42is returned to stripping tower 7 by fan 25 as a part of 9a carrier gasafter the temperature of the gas was raised with pre-heater 40, whennecessary.

[0097] Even in this example, it is possible to adjust the concentrationof NH₃ and lower the concentration of NO in the gas at the outlet ofreducing catalyst tower 42 by adjusting the concentration of the NH₃ inthe gas at the outlet of tower 42 with valve 44 based on the indicatedvalue of ammonia meter 43 placed near the outlet of reducing catalysttower 42. Also, the amount of the energy necessary for heating a gas andliquid can be decreased by circulating the gas discharged from theoutlet of reducing catalyst tower 42 to stripping tower 7.

[0098] In this connection, it is preferable to use an NH₃ decomposingcatalyst 13 as showen in FIG. 4 having a denitrating function since twocatalyst towers are required and it is necessary to adjust the amount ofNH₃ necessary for reducing NO, by the opening of valve 44 based on theindicated value of ammonia meter 43 placed near the outlet of reducingcatalyst tower 42 in this example.

[0099] Next, a specific example in which the catalyst device as shown inFIG. 9 was used is described.

EXAMPLE 4

[0100] In the device as shown in FIG. 9, a catalyst which was preparedby the same method for preparing a catalyst as that used in Example 1with the exception that 20 kg of a first component of catalyst and 202 gof a second component of catalyst were used and thus the ratio of thesecond component to the first component (the second component/the firstcomponent) was changed to {fraction (1/99)} (in this case, Pt contentcorresponds to 1 ppm, excepting the weight of a catalyst substrate andinorganic fibers) was used as NH₃ decomposing catalyst 13A having adenitrating function. Likewise, NH₃ decomposing catalyst 13B having a Ptcontent of 5 ppm and comprising 20 kg of a first component and 404 g ofa second component mixed therein was prepared by the same method forpreparing a catalyst as that described in Example 1. A test for treatingan effluent was conducted by using catalyst tower 12 as shown in FIG. 9under the conditions shown in Table 3. The relation between theconcentration of the sum of NOx and N₂O, and the reaction temperature atthe time of starting the test is shown by curve (a) in FIG. 10. TABLE 3Item Condition Rate of treating effluent 1.6 L/h Amount of NH₄^(+ in effluent) 2,000 mg/L Gas flow rate at inlet of 1.3 m³/h catalystlayer Gas composition NH₃: 10,000 ppm H₂O: 30% Air: the remainderTemperature 350° C. Areal velocity 10 m/h

COMPARATIVE EXAMPLE 2

[0101] A test for treating effluent was conducted under the sameconditions as those in Example 4 with the exception that only catalyst13B was used as catalyst. The result thus obtained is shown by curve (b)in FIG. 10.

[0102] From FIG. 10, it can be understood that the concentration of thesum of NOx and N₂O can largely be reduced in the test of Example 4compared with the test in Comparative Example 2.

EXAMPLE 5

[0103] A test for treating an effluent was conducted by using the samecatalyst under the same conditions as those used in Example 4 with theexception that a mordenite having iron supported thereon was used inplace of catalyst 13B. The relation between the concentration of the sumof NOx and N₂O, and the reaction temperature when a test for treating aneffluent was conducted by using the catalyst of this example under thesame conditions as in Example 4 is shown by curve (c) in FIG. 10. Aswill be understood from FIG. 10, the concentration of the sum of NOx andN₂O can further be reduced according to this example than theconcentration obtained in Example 4 since a mordenite having ironsupported thereon has a function of decomposing N₂O.

[0104] Whereas catalyst layer 13A and catalyst 13B each having adifferent composition are arranged in series in the embodiment shown inFIG. 9, plural catalyst layers can be arranged in parallel in a catalystdevice as shown in FIG. 11. Thus, it is possible to increase NH₃decomposition ratio to a high level and suppress the NOx concentrationand N₂O concentration in the gas at the outlet of a catalyst tower to alow level by alternately arranging plate-like catalysts comprisingcatalyst A or catalyst C (a catalyst having a function of decomposingN₂O) in a catalyst device as shown in FIG. 11. Further, in the casewhere the length of a catalyst layer cannot be extended by constraintsfrom an apparatus, a method in which a device as shown in FIG. 11 isused is effective. It is also effective to dispose catalysts of anothershape such as a honeycomb shape in a catalyst device as shown in FIG.11.

[0105] According to the embodiments as shown in FIG. 9 and FIG. 11, theproblem that when the NH₃ concentration in the gas once-treated in acatalyst tower was reduced to lower than a prescribed value, NOxconcentration and N₂O concentration in the gas at the outlet of acatalyst tower become slightly high is resolved, and the amounts ofhazardous substances produced can considerably be reduced.

INDUSTRIAL APPLICABILITY

[0106] The present invention can be applied to the treatment ofeffluents containing an ammonia nitrogen at a high concentration such asan effluent discharged from a thermal power plant, and the ammonia canbe removed from the effluent at a high efficiency while reducing theamount of N₂O or the like formed at that time.

1. A method for treating an ammonia-containing effluent comprising astripping step in which the ammonia (NH₃) contained in theNH₃-containing effluent is transferred with a carrier gas from theeffluent into a gas phase, a step for adding an oxygen-containing gas tothe NH₃-containing gas produced at the stripping step, and an NH₃decomposing step in which the oxygen-containing gas and theNH₃-containing gas are contacted with one or more kind of catalysts usedfor decomposing NH₃, at a prescribed temperature to decompose the NH₃into nitrogen and water, the concentration of oxygen in the gas mixtureintroduced into the NH₃ decomposing step being adjusted.
 2. The methodfor treating an NH₃-containing effluent according to claim 1 wherein themethod further comprises a step by which a part of the gas resulted inthe NH₃ decomposing step is discharged outside the effluent treatingsystem and the remaining part of the gas is recycled as a part of thecarrier gas to be used in the stripping step.
 3. The method for treatingan NH₃-containing effluent according to claim 1 or 2 wherein theconcentration of oxygen in the gas mixture to be introduced into the NH₃decomposing step is adjusted to a value within the range of 2 to 15%. 4.The method for treating an NH₃-containing effluent according to any oneof claims 1 to 3 wherein the concentration of oxygen in the gas mixtureto be introduced into the NH₃ decomposing step is adjusted so that theconcentration of the N₂O in the gas resulted in the NH₃ decomposing stepbecomes a value within a prescribed range.
 5. The method for treating anNH₃-containing effluent according to any one of claims 1 to 4 whereinthe catalyst used for decomposing NH₃ comprises a first component havingan activity of reducing nitrogen oxides (NOx) with NH₃ and a secondcomponent having an activity of forming nitrogen oxides from NH₃.
 6. Themethod for treating an NH₃-containing effluent according to any one ofclaims 1 to 5 wherein the catalyst used for decomposing NH₃ comprises,as a first component, an oxide of titanium (Ti) and an oxide of one ormore elements selected from the group consisting of tungsten (W),vanadium (V), and molybdenum (Mo), and, as a second component, a silica,zeolite, and/or alumina having one or more noble metals selected fromthe group consisting of platinum (Pt), iridium (Ir), rhodium (Rh), andpalladium (Pd) supported thereon.
 7. The method for treating anNH₃-containing effluent according to any one of claims 1 to 5 whereinthe catalyst used for decomposing NH₃ is a zeolite or comprises, as amain component, a zeolite.
 8. The method for treating an NH₃-containingeffluent according to any one of claims 1 to 4 wherein the concentrationof oxygen in the gas mixture to be introduced into the NH₃ decomposingstep is adjusted so that the concentration of the NH₃ remaining in thegas resulted in the NH₃ decomposing step becomes a value within aprescribed range, while using the concentration of the NH₃ in the gas asan index, instead of the concentration of the N₂O in the gas.
 9. Themethod for treating an NH₃-containing effluent according to claim 8wherein the concentration of the NH₃ remaining in the gas resulted inthe NH₃ decomposing step is higher than 50 ppm.
 10. The method fortreating an NH₃-containing effluent according to claim 8 or 9 whereinthe gas pressure in the effluent treating system is controlled to aprescribed value so that the amount of a part of the gas resulted in theNH₃ decomposing step and discharged outside the system becomes equal tothe increase of the total amount of the gas including the amount of theoxygen-containing gas supplied into the system.
 11. The method fortreating an NH₃-containing effluent according to any one of claims 8 to10 wherein the method further comprises a step for removing ammonia froma part of the gas resulted in the NH₃ decomposing step after the part ofthe gas was discharged outside the system.
 12. The method for treatingan NH₃-containing effluent according to any one of claims 1 to 4 whereinthe step for decomposing the NH₃ into nitrogen and water by contactingthe NH₃-containing gas with a catalyst is a step in which two or morekind of catalysts each having a different oxidizing power are used, andthe NH₃-containing gas is contacted first with a catalyst having arelatively low NH₃ oxidizing power to decompose a part of the NH₃ intonitrogen and water and then with a catalyst having a relatively high NH₃oxidizing power to decompose the remaining part of the NH₃ into nitrogenand water, or a step in which the NH₃-containing gas is contacted at thesame time with two or more kind of the catalysts to decompose the NH₃into nitrogen and water.
 13. The method for treating an NH₃-containingeffluent according to claim 12 wherein two or more kind of the catalystseach having a different power for oxidizing NH₃ comprise, as a firstcomponent, an oxide of titanium (Ti) and an oxide of one or moreelements selected from the group consisting of tungsten (W), vanadium(V), and molybdenum (Mo), and, as a second component, a silica, zeolite,and/or alumina having one or more noble metals selected from the groupconsisting of platinum (Pt), iridium (Ir), rhodium (Rh), and palladium(Pd) supported thereon, and the power for oxidizing NH₃ is adjusted bythe ratio of the content of the first component to that of the secondcomponent.
 14. The method for treating an NH₃-containing effluentaccording to claim 12 wherein the catalyst having a relatively high NH₃oxidizing power is a zeolite.
 15. An apparatus for treating anNH₃-containing effluent comprising a stripping means for transferringthe ammonia (NH₃) contained in the NH₃-containing effluent with acarrier gas from the effluent into a gas phase, a means for adding anoxygen-containing gas to the NH₃-containing gas produced in thestripping means, an NH₃ decomposing means by which the oxygen-containinggas and the NH₃-containing gas are contacted with one or more kind ofcatalysts used for decomposing NH₃, at a prescribed temperature todecompose the NH₃ into nitrogen and water, and a means for adjusting theconcentration of oxygen in the gas mixture to be introduced into the NH₃decomposing means.
 16. The apparatus for treating an NH₃-containingeffluent according to claim 15 wherein the apparatus further comprises ameans for determining the concentration of N₂O or NH₃ in the gasdischarged from the NH₃ decomposing means and controlling theconcentration to a value within a prescribed range.
 17. The apparatusfor treating an NH₃-containing effluent according to claim 15 or 16wherein the catalyst used for decomposing NH₃ comprises, as a firstcomponent, an oxide of titanium (Ti) and an oxide of one or moreelements selected from the group consisting of tungsten (W), vanadium(V), and molybdenum (Mo), and, as a second component, a silica, zeolite,and/or alumina having one or more noble metals selected from the groupconsisting of platinum (Pt), iridium (Ir), rhodium (Rh), and palladium(Pd) supported thereon.
 18. The apparatus for treating an NH₃-containingeffluent according to claim 15 or 16 wherein the catalyst used fordecomposing NH₃ is a zeolite or comprises, as a main component, azeolite.
 19. The apparatus for treating an NH₃-containing effluentaccording to claim 15 wherein the catalyst used for decomposing NH₃ is acatalyst in which one or more catalyst layers having a relatively lowNH₃ oxidizing power and one or more catalyst layers having a relativelyhigh NH₃ oxidizing power are arranged in series, or a plate-likecatalyst in which catalyst layers having a relatively low NH₃ oxidizingpower and catalyst layers having a relatively high NH₃ oxidizing powerare arranged alternately in the direction perpendicular to the directionof the gas flow with the surfaces of the layers being held in parallelto the gas flow direction.